Because of its high refractive index, titanium dioxide is used as a high-quality pigment in many sectors, e.g. plastics, coatings, paper and fibres. However, titanium dioxide is photoactive, meaning that undesired photocatalytic reactions occur as a result of UV absorption, leading to degradation of the pigmented material [The Chemical Nature of Chalking in the Presence of Titanium Dioxide Pigments, H. G. Völz, G. Kaempf, H. G. Fitzky, A. Klaeren, ACS Symp. Ser. 1981, 151, Photodegradation and Photostabilization of Coatings].
In this context, titanium dioxide pigments absorb light in the near ultraviolet range, the result being that electron-hole pairs are produced, which lead to the formation of highly reactive radicals on the titanium dioxide surface. The radicals produced in this way result in binder degradation in organic media. It is known from experimental investigations that hydroxyl ions play a dominant role in the photocatalytic process [Photocatalytic Degradation of Organic Water Contaminants: Mechanism Involving Hyroxyl Radical Attack, C. S. Turchi, D. F. Ollis, Journal of Catalysis 122, 1990, 178-192].
It is known that the photoactivity of TiO2 can be reduced by doping the TiO2 particles (e.g. with aluminium) or by means of inorganic surface treatment (e.g. by coating with oxides of silicon and/or aluminium and/or zirconium) [Industrial Inorganic Pigments, ed. by G. Buxbaum, VCH, New York 1993, Seite 58-60]. In particular, several patents describe the application of the most dense possible, amorphous layer of SiO2 to the particle surface, this being known as a “dense skin”. The purpose of this skin is to prevent the formation of free radicals on the particle surface.
Wet-chemical methods for production of a dense SiO2 skin, and of a further Al2O3 coating on inorganic particles, particularly on TiO2, are described in U.S. Pat. No. 2,885,366, U.S. RE. 27,818 and U.S. Pat. No. 4,125,412. EP 0 245 984 B1 indicates a method which, as a result of simultaneous addition of a solution containing Na2SiO3 and a solution containing B2O3, can be performed at relatively low temperatures of 65 to 90° C.
SiO2 dense-skin treatments are also carried out in order to increase the abrasion resistance of glass fibres coated in this way and reduce the slipping properties of the fibres in the products manufactured. In this connection, U.S. Pat. No. 2,913,419 describes a wet-chemical method in which silicic acid is precipitated onto the particle surface together with polyvalent metal ions, such as Cu, Ag, Ba, Mg, Be, Ca, Sr, Zn, Cd, Al, Ti, Zr, Sn, Pb, Cr, Mn, Co, Ni.
The method according to US 2006/0032402 makes it possible to increase the photostability of dense-skin TiO2 pigments. It is based on the incorporation of Sn, Ti or Zr in the SiO2 skin applied by a wet-chemical process.
In addition to the known wet-chemical methods for coating the surface of TiO2 particles, there are also dry-chemical methods in which the dense SiO2 skin is deposited from the gas phase. In this case, during titanium dioxide production by the chloride process, a silicon compound, preferably SiCl4, is added to the TiO2 particle stream with a temperature of over 1,000° C., such that a uniform, dense SiO2 layer is formed on the particle surface.
EP 1 042 408 B1 describes a gas-phase method for surface coating with Si and B, P, Mg, Nb or Ge oxide.